In 2.8 how do I drop a modeling tool?
There is always one tool active so closest option to exiting tool is to switch to Select Box.
For shortcut:
You can press first 'Escape' key to cancel if in middle of operation and then 'W' key once to switch out of current tool like extrude/inset etc. to Box select tool.
Pressing 'W' second time or more will change the select option from box to circle and so on.